The present invention relates in general to the running of municipal wastewater treatment plants on the basis of indications relating to pollution loads measured at the inlet of the treatment station, in the form of conductivity, turbidity and flow rate values: a practical illustration of this is given in the case of a treatment station using activated sludge for the physicochemical dephospatization of raw water simultaneously with purification by means of activated sludge, the objective of this invention being to define automated management rules for the addition of dephosphatizing reactants (ferric chloride) into the raw water proportional to the pollution load entering the station for purifying the said raw water.
It is known that, to achieve this objective, it is necessary, on the one hand, to estimate, in line, the total phosphate load of the raw water to be treated and, on the other hand, to evaluate the yield of the dephosphatization reaction. In this way it is possible to determine the amount of dephosphatizing reactants to be added to the raw water, this amount having to be sufficient to remove the phosphorus so as to meet the imposed discharge standards, while still controlling the excess reactants so as to optimize the costs and avoid unjustified overproduction of additional sludge.
At the present time, this estimation is performed continuously by means of analysers, the investment costs of which are high and the running and maintenance of which are stringent.
The present invention is based on the discovery that, for municipal wastewater, any pollution load (whether carbon load, nitrogen load, phosphorus load, etc.) that has to be treated may be estimated from simple measurements of the flow rate conductivity and turbidity of the raw water.
The present proprietor has studied, for example, the relationship which exists between the total phosphorus load and:
the conductivity of the raw water to be treated;
the turbidity of the raw water; and
the flow rate of the raw water.
It should be pointed out that the invention is not limited to the treatment of raw water and that the use of load indicators is appropriate for controlling any purification phase in the treatment line.
For a given municipal wastewater, not containing more than 30% of its pollution flux originating from industrial discharges, it is accepted that the composition matrix of the water is relatively constant since the pollution source is essentially of domestic origin and comes from the users connected to the sewerage system.
DE-A-4 006 689 describes a method of managing water treatment plants based on measurements of the conductivity and turbidity of the water to be treated.
Starting from the abovementioned observations and prior art, the present invention provides a method which consists, based on continuous measurements of the conductivity and turbidity of the raw water to be treated, in carrying out the following operations:
estimating the concentration of pollutants in the raw water, on the basis of the said conductivity and turbidity values thus measured;
continuously measuring the flow rate of the raw water to be treated;
calculating the load of pollutants in the raw water as a function of its flow rate; and
determining downstream treatment actions to be taken to eliminate the estimated total load of the polluting component.
As will have been understood, the method forming the subject matter of the present invention aims to determine a pollutant concentration equivalent which is a function of the said pollution indicators, that is to say of the continuously measured conductivity and turbidity values. Measuring the flow rate of the raw water to be treated then makes it possible to determine the pollutant load from this estimated concentration.
According to the present invention, the polluting component to be eliminated may be an orthophosphate, a metaphosphate or a polyphosphate, or in general an organic or inorganic chemical compound containing phosphorus. This polluting component may also be a nitrate, or in general an organic or inorganic chemical compound containing nitrogen, or else an organic or inorganic chemical compound containing carbon.
According to the present invention, the relationship between the estimated concentration and the pollution indicators associates:
the conductivity with the dissolved pollution which, in the case of a pollution load consisting of phosphorus, is essentially composed of orthophosphates. Conductivity ranges are therefore associated with factors equivalent to the concentration of soluble phosphorus components;
the turbidity with the colloidal and particulate pollution, this pollution consisting of various components, including phosphorus, which relate to suspended matter. Turbidity ranges are therefore associated with factors equivalent to the concentration of insoluble phosphorus components.